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<title>DATABASE: Reactions with Gases</title>
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<h3>Reactions with Gases:</h3>

<p>Any species with a name ending with <nobr>&#147;(g)&#148;</nobr> is
considered to be a gas.</p>

<p>The <i>fugacity</i> for a gas is equivalent to the activity for an aqueous species.</p>

<p><b>log<i>K</i>&deg;-value:</b> for an equilibrium between a gas phase and an
aqueous solution, it must be assumed either <nobr><b>1)</b>&nbsp;constant</nobr>
pressure or  <nobr><b>2)</b>&nbsp;constant</nobr> volume. The value of
<nobr>log<i>K</i>&deg;</nobr> will be different for these two cases. Example:</p>

<p class="indent">
<nobr>CO<sub>2</sub>(g)</nobr> <nobr>+&nbsp;H<sub>2</sub>O</nobr>
	<img src="images/rlh.gif" alt="=" border="0" width="14" height="10">
	<nobr>HCO<sub>3</sub><sup>&#8722;</sup></nobr> <nobr>+&nbsp;H<sup>+</sup></nobr>
  </p>

<p>For constant pressure:</p>
<p class="indent">
<nobr>log<i>K</i><sub>p</sub>&deg;</nobr> = <nobr>log [HCO<sub>3</sub><sup>&#8722;</sup>]</nobr>
	<nobr>&#8722; pH</nobr> <nobr>&#8722; log <i>P</i><sub>CO<sub>2</sub></sub></nobr>
  </p>

<p style="margin-top:0.5ex;">
<i>P</i> is in units of bar.  In the Earth&rsquo;s atmosphere
<nobr>log&nbsp;<i>P</i><sub>CO<sub>2</sub></sub></nobr>&#8776;<nobr>&#8722;3.5</nobr></p>

<p>For constant volume:</p>
<p class="indent">
<nobr>log <i>K</i><sub>v</sub>&deg;</nobr> = <nobr>log [HCO<sub>3</sub><sup>&#8722;</sup>]</nobr>
	<nobr>&#8722; pH</nobr> <nobr>&#8722; log [CO<sub>2</sub>]</nobr>
  </p>

<p class="cont">where the concentrations are given in <nobr>mol/L.</nobr></p>

<p>Using the eqn. for perfect gases <nobr>(<i>P V</i></nobr> = <nobr><i>n
<a href="1_Some_Constants.htm">R</a> T</i>)</nobr> it is found that:</p>

<p class="indent">
<nobr><i>P</i><sub>CO<sub>2</sub></sub></nobr> = <nobr>[CO<sub>2</sub>]</nobr> 0.01 <nobr><i>R T </i></nobr>
  </p>

<p>The factor 0.01 is needed to convert the units <nobr>m<sup>3</sup></nobr> to L and Pa to bar.
Finally one obtains:</p>

<p class="indent">
<nobr>log <i>K</i><sub>p</sub>&deg;</nobr> = <nobr>log <i>K</i><sub>v</sub>&deg;</nobr>
	<nobr>+ log (0.01 <i>R T</i>)</nobr>
  </p>

<p><b>Important:</b> DATABASE and SPANA are based on
<nobr>log<i>K</i><sub>p</sub>&deg;</nobr> values for gases, with a standard state of 1&nbsp;bar
<nobr>(&#8776;1 atm).</nobr></p>

<p>During the calculations to make a diagram, the concentration of a gas is not evaluated.
The fugacity is calculated and displayed in &#147;log activity&#148; diagrams.</p>

<p><b>Example</b> of a <u>redox</u> gas reaction:</p>

<p class="indent">
<nobr>2&nbsp;H<sub>2</sub>O</nobr>
	<img src="images/rlh.gif" alt="=" border="0" width="14" height="10"> 
	<nobr>4&nbsp;H<sup>+</sup></nobr>  <nobr>+&nbsp;4&nbsp;e<sup>&#8722;</sup></nobr> <nobr>+&nbsp;O<sub>2</sub>(g)</nobr>
  </p>

<p class="cont">line in database:</p>

<p class="indent">
<nobr><tt>O2(g);-83.12;;;H+;-4;;;e-;-4;H2O;2;;;;;-4;Wateq4F</tt></nobr>
  </p>

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